The importance of detecting leaks in fuel tanks has long been recognized by the automotive industry. Unfortunately, present methods of detecting leaks in fuel tanks are either not suited for the production line methods of the automotive industry or are not sufficiently reliable. The objective of this invention is to provide a method and apparatus for detecting leaks in a fuel tank that are highly accurate and reliable and can be utilized under factory conditions.
The present method of fuel tank leak detection used in the automotive industry is the "bubble" method. A fuel tank is submerged in water, pressurized, and then examined to see if bubbles of air escape from the tank. There are several major disadvantages to this method, the most important of which are the speed and reliability of the test. In operation, the speed and reliability of the bubble method are inexorably connected. The bubble test is conducted manually whereby a human operator will watch for bubbles arising from the submerged fuel tank. The accuracy of the test therefore, depends on the alertness of the operator of the test. The human element of this test cannot readily be eliminated, as it is difficult to have an automated unit detect bubbles arising from the tank. In addition, the bubble test is limited by its sensitivity, only bubbles of a volume of 0.04 cubic centimeters or larger are noticeable by the human eye and, therefore, smaller leaks cannot be detected. While this method has its advantages, such as flexibility of the test in that the observer may ignore leaks that are irrelevant such as from defective seals applied to the fuel tank for testing, they are outweighed by the disadvantages. Even the flexibility is adversely affected by the time constraints of the automotive industry. The faster the operator is forced to conduct the test, the less accurate he is likely to be. It is also necessary to submerge the tank in water, thereby causing the tank to be wet and requiring a drying cycle before the tank may be painted or finally assembled.
Other methods of leak detecting include mass spectrometer pressures tests and an air pressure decay test. A mass spectrometer leak test involves placing the test part in a leak tight enclosure and sealed at all ports. A high vacuum is drawn either inside the part or inside the enclosure and helium is introduced into the other which is left at atmospheric pressure. Any helium detected by a spectrometer scanning the high vacuum space indicates a leak. Although this test is highly sensitive, may be easily automated and can be operated quickly, it is a delicate system that it not suited for use in the automotive industry. Another major disadvantage is the necessity to maintain a high vacuum which require special parts and special maintenance. The result of the exotic equipment is the relatively high cost of mass spectrometer testing. In contrast, the air pressure decay test is inexpensive and well suited for automotive industry conditions. However, the test is neither fast enough nor accurate enough for the automotive industry. The test is conducted by either pressurizing or evacuating the fuel tank to be tested. Either the rate of pressure differential is measured or the rate of flow necessary to maintain a constant pressure is measured. The speed of this test is greatly affected by the part size and by the test pressure, and the accuracy is affected by the temperature of the part tested.
A rather comprehensive and descriptive work on leak detection methods has been prepared by Varian Associates Incorporated and is entitled "Introduction to Helium Mass Spectrometer Leak Detection", published in 1980.
Halogen leak detectors are commonly used, particularly in the refrigeration industry, and typically involve system pressurized with a test gas containing an inorganic halide or similar gases. The exterior of the system is then scanned with a sniffer probe sensitive to traces of the test gas. Disadvantages of the system are that such leak detectors will respond to a variety of other gases, including cigar smoke, and that most systems release the test gas into the atmosphere.
A particular problem found in leak detection systems that utilize a test gas is the discharge of the test gas into the atmosphere both outside the plant and inside the plant. The EPA has established allowable exhaust concentrations for dichlorodiflouromethane (R-12) of 1000 parts per million. Although R-12 is not toxic, it may cause oxygen deficiency at concentrations of approximately 28.5 percent by volume or 89.6 pounds per thousand cubic feet of air. These concentrations may appear at first to be quite high and improbable, but consistent and continuous exposure to significant concentrations of halogens may cause an affect upon the operator of the test. It is therefore a particular object of the invention to reduce the chances of such an occurrence.
The objective of this invention is to provide a rapid yet accurate method of detecting leaks in fuel tanks suitable for use in the automotive industry. Systematic testing of fuel tanks during production serves two purposes, it ensures the proper functioning of the production system and it ensures quality control by eliminating tanks that would leak in use. The system must be rugged enough to withstand the automotive factory floor conditions, particularly the rapid loading and unloading required for production line speed, and yet be sensitive enough to detect small leaks in a fuel tank. Most leaks in fuel tanks occur around the welding seam of the tank, as most fuel tanks are of a two-piece metal construction. Recently, however, seamless plastic fuel tanks have been growing in popularity with the automotive industry. Small, but significant leaks may occur in plastic tanks at any portion of the tank. In spite of the automotive industry's desire for accuracy in leak detection, the overriding consideration has always been the speed with which a test may be run. Each second may be critical in the efficiency operation of an automotive plant, making automation of the system a necessity.
Perhaps the most important requirement of a leak detecting system in the automotive industry is its ability to be reused time and time again as the test is repeated with the numerous individual parts to be tested. A detection system must retain its sensitivity and consistently produce accurate results. The present invention particularly avoids many problems which could arise from such repeated use.
In selecting a leak detection system that utilizes a vacuum, there are again competing interests. The smaller the chamber to be evacuated, the more quickly and efficiently the system can be run. Servicing may be a crucial factor in the selection of leak detecting system, as any downtime caused by a wait for a needed part can be very costly in today's automotive industry. Another problem faced in leak detection systems is the variation in a group of parts to be tested. Although the parts may have been simultaneously or consecutively manufactured, the products may not be identical and may have small variations in their dimensions or configurations. Accordingly, a leak detection system must be able to be adapted to the individual part being tested.
Another problem that must be confronted in utilizing vacuum leak detection systems is the effect of the resulting pressure differential upon the fuel tank being tested. Conventional fuel tanks are made of a two-piece metal construction welded at the flange of each half. Since leaks often occur in fuel tanks at the weld at the flange, one method previously used in the art utilizes a flexible chamber that will collapse around the tank being tested at its weaker points. Thus, the flange of the tank is unsupported through the test, but the weaker basins of the tank halves are supported by the vacuum chamber itself.
Although flexible chambers are useful in reducing the volume in the chamber needed to be evacuated, the use of a flexible chambers reduces the effectiveness of other methods which improve the speed and accuracy of the test. Additionally, flexible chambers will tend to block or cover leaks and therefore reduce the effectiveness of the test except at the area surrounding the flange. As previously noted, this is not as great of a concern when conventional metal tanks are tested as the majority of leaks occur at the seams; seamless plastic fuel tanks, however, are likely to leak at any given point on the tank. Even with conventional metal tanks, uniform leak detection is an objective.
Furthermore, a flexible chamber does not provide a tank with structural support from the atmospheric pressure without the chamber. Should a vacuum be drawn within the tank to equalize the pressure within the tank and within the chamber, as may be done in scavenging the tank, a flexible chamber would allow the external air pressure to act upon the tank. As fuel tanks have much greater tensile strength than compressive strength, it is important that if even a slight vacuum occur within the tank that the chamber be sufficiently rigid to withstand atmospheric pressure. Moreover, flexible chambers cause very great difficulty in supporting the peripheral seals and very great difficulty in effecting chamber modifications due to difficulty in effective gluing of elastic material.
A common method of supporting parts to be tested by vacuum chamber leak detection methods is to provide the test chamber with ribs, so that the part will be suspended between ribs when the chamber is closed. In this manner, a substantial portion of the surface of the part tested is exposed to the vacuum created within the chamber. A particular problem with such devices, however, is that the portions of the surface of the part tested that are contacted by the ribs are not exposed to the vacuum. Any leaks occurring in such portions not exposed to the vacuum would not be detected in the test as the ribs would effectively be "plugging" the leak. It is an objective of the invention to provide support for the part tested while completely exposing the entire surface of the part tested to the vacuum in a vacuum leak detection system.
Detecting the presence of a leak is important to ensure that no container with a leak is released to the public, but unless the leak is detected, the container cannot easily be repaired nor may the cause of the leak cannot be determined. Therefore, it is a distinct advantage for a leak detection method or apparatus to provide a means for locating any leak detected. Knowledge of where the leak in a container is located could aid in identifying deficiencies in the material used in making the container, in the method of forming the container and in any method of sealing the container, such as welding. Additionally, locating any leak indicated by a leak detection system would expose any deficiencies in the leak detection system itself and prevent the waste of a good container which may have been rejected merely because of a leak in the detection system. It is an objective of the invention to locate leaks in addition to detecting leaks.
Other objects, features and advantages of the present invention will become apparent from the susequent description and the appended claims taken in conjunction with the accompanying drawings.